Piezo-electric device



v 1,617,995 I A. L. R. ELLIS PIEZO ELECTRIC DEVICE Filed July 14, 1925 Im/entor: Alvarado LEE His,

H is Attorney.

Patented Feb. 1927.

warren stares PATENT or 1,617,995 FlCE.

ALVARADO L. R. ELLIS, OF SWAMPSCOTT, MASSACHUSETTS, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

' rmzo-amcrarc DEVICE.

Application filed July 14, 1925. Serial No. 43,628.

Myinvention relates to piezo-electric devices, and has for its principal object the provision of an arrangement and method whereb the free vibration period of the piezo-e ectric member comprised in such a device is maintained substantially constant irrespective of changes in the conditions under which it is operated.

The free vibration period of a piezo-electric oscillator, such as a 'slab or disk of quartz for example, is determined for the most part by the dimensions of the slab but is affected to some extent by the conditions under which the oscillator is operated. Thus the thermal expansion of crystalline quartz, though slight, is appreciable and the velocity of sound in this material varies slightly with temperature. These combined eiiects are not serious at low frequencies but become of great importance at frequencies of two million cycles or more. It so happens that the utilization ofthe quartz oscillator is most desirable in this range of frequencies. While the maintenance of constant temperature in the laboratory is not a diflicult matter, it is a serious problem under ordinary operating conditions. In accordance with my invention, this difiiculty is avoided by the provision of means operable in response to temperature changes to vary the spacing between the electrodes and the oscillator slab in a manner to maintain the resonance frequency of the slab substantially constant irrespective of temperature changes.

My invention will be better understood from the following description when considered in connection with the accompanying' drawing and its scope will bepointed out in the appended claims.

Referringto' the drawings, Fig. 1 shows a view of an oscillator constructedin accordance with my invention ;vFi 2 shows a plan view of the oscillator wit its cover removed; and Fig. 3 shows a modification thereof; 1

Figs. land 2 show a quartz crystal 1 supported upon an electrode 2 which is mounted on an insulation base 3 and is connected to a terminal 4 through means shown as a screw 5. An electrode 6 is spaced from the crystal 1 and base 3 by means of a conductive ring 7 connected to the terminal 8 through a screw 9. While the device is shown as circular in form, it will be apparent'that it may be of any suitable shape.

In order'to simplify the drawing, the spacing between the oscillator 1 and electrode 6 has been shown as of considerable extent. It should be "noted, however, that in an actual construction this spacing is only a few thousandths of an inch.

Assuming for the purpose of explanation that the crystal 1 is suspended in air so that it can be made to oscillate freely, it will be readily understood that a layer of air adjacent the oscillating surfaces of the crystal will vibrate just as though it formed an integral part of the crystal. The resonance frequency of the crystal under these conditions depends to a large extent on the dimensions of the quartz slab and to a lesser extent on the mass of air which vibrates in Variation in the resonance frequency with temperature changes is likely to be accentuated by the mounting and casing, of the. oscillator. Thus with the usual and convenient construction shown by Fig. 1, any change in the thickness of the slab 1 due to changes in temperature causes variation in the spacing between the oscillator l and electrode 6, thereby forcing the air into and out of the space surroundin the oscillators 1 and electrode 2 and changm the loading and resonance frequency of t e oscillator. Expansion-and contractlon of the electrode 2 and ring 7. are likely to produce further changes of this character.

I propose to avoid these difficulties by constructing the supporting electrode 2 and ring 7 of materials having such a combined copper and invar layers of such thickness that their combined thermalexpansion is equal to that of the oscillator. Where it is desired to compensate for changes. in the density and elasticity of the air due to variation in temperature, the ring 7 or elec trode 2 may be made of a material or materials having a coeliicient of thermal expansion adapted, to change the spacing between the electrode 6 and oscillator 1 in the proper manner.

As indicated in Fig. 3, where'direct expansion and contraction of the supporting electrode 2 or ring 7 is insufiicient to produce the proper degree of compensation, bimetallic strips 10 similar to those commonly utilized in thermostats are interposed between the insulation base 3 and electrode 6. While the electrode 6 has been shown as adapted to be moved for purposes of temperature compensation, it will bereadily understood that the same effect may be produced by moving either the insulation base 3 or both the insulation base 3 and the electrode 6'with respect to the oscillator 1.

The embodiments of the invention illustrated and described herein have been selected for the purpose of clearly setting forth the principles involved. It will be apparent, however, that the invention is susceptible of being further modified to meet the different conditionsencountered in its use and I therefore aim to cover by the appended claims all modifications within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. The combination of a piezo-electric oscillator, a support for said oscillator, an

electrode adjacent a surface of said oscillator, and thermal responsive means lnterposed between said support and electrode whereby the spacing between said electrode and surface is controlled to maintain the resonance frequency of. said oscillator constant irrespective of' variation in tempera- I ture. I n

2. The combination of a piezo-electric oscillator, an electrode adjacent a surface of said oscillator, and an electrode support having a co-eflicient of thermal expansion which is substantially the same as that of said oscillator. 1

3. The combination of a plurality of electrodes, a piezo-electric oscillator interposed between said electrodes, and a thermal responsive support for one of said electrodes whereby the spacing between said electrodes is controlled 'to maintain the resonance frequency of said oscillator constant irrespective of variation in temperature.

4:; The combination of a pair of electrodes, a quartz slab interposed between said electrodes, and a thermal responsive support for one of said electrodes whereby the spacing between said electrodes is controlled in accordance with the temperature of the ambient air. I

5. The combination of a pair of elec-' trodes, a piezo-electric slab interposed between said electrodes, and a bi-metallic strip arranged to control the spacing between said electrodes in accordance with variation in ALVARADO L. R. ELLIS. 

